Many industrial processes require large quantities of low grade heat which are conveniently and conventionally supplied in a form of low pressure steam. An industrial plant carrying out such processes, such as an oil refinery, continuously produces high pressure steam for a power plant that generates electricity, and obtains low pressure steam from the exhaust of the turbine power plant. Generally, the installed electrical load of an industrial plant greatly exceeds its generating capacity; and the plant contracts with a local power utility for its remaining power. The utility is obligated to furnish power up the installed electrical load of the industrial plant, or to the level established by agreement with the plant, even if such load occurs infrequently with time for only a few hours a day. Consequently, the utility must have generating capacity available for use when the industrial plant calls for its contracted power.
Because the peak power demands on a utility occur infrequently, a utility builds and regularily operates its most efficient plant to supply the baseload of the customers connected to its grid; and supplies intermediate loads, greater than the base load with less efficient plants. Peak power is supplied by operating diesel or gas turbine generating plants for very short intervals of time. These peak power generating units quickly can be brought onto and removed from the line and are advantageous from this standpoint. However, they are relatively inefficient and hence more expensive to operate than either the base or intermediate load power plants, such as coal or oil burning plants. Consequently, the cost of peak power is several times that of what is termed baseload power.
For a large industrial plant, such as oil refinery, the load requirements are so high, and the actual equipment to supply such load so complex and costly, that the plant is charged a fixed annual premium in order for the utility to be able to guarantee that it can deliver to the plant either 100% of its installed load or a predetermined portion thereof reached by agreement. Thus, the electrical bill of a large industrial plant has two components: a variable charge based on actual energy consumed, and a fixed annual charge based on the installed electrical capacity of the plant or the peak power that the utility is obligated to furnish on demand to the plant. The fixed annual charge as high as $100 per kW.
In order to reduce its annual fixed charge, a plant may install a standby power generating system based on a diesel driven generator whose operation is assured when the power being supplied by utility reaches the level predetermined by agreement between the utility and the plant. The switching in of standby generating capacity for supplying "peak power shaving" is cost effective after the fixed change reaches a threshhold. Computerized power saving techniques are presently in use for continuously monitoring the connected electrical load of a plant. These techniques arrange for hierarchical shutdown of the plant as the connected load increases beyond the total commited by the utility; for example, lighting loads are usually shed first. Thus, the peak power requirement of an industrial plant is of considerable economic importance in planning the operation of the plant.
Installation of energy saving systems in such a plant in order to convert low grade heat, normally wasted, to electrical power for the plant is often economically justified on the basis of the savings afforded by the variable charge for the energy generated using the low grade heat. This approach to energy saving does not address the primary problem of the fixed annual charge based on the installed capacity of the plant. For this reason, there is often hesitancy of the part of some plants to make the large capital investment required to convert low grade heat into energy.
It is therefore an object of the present invention to provide a new and improved method of and means for peak power shaving using low grade heat which permits a plant having low grade heat available to reduce its fixed annual charges at reduced initial investment.